Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine and an electric motor to provide the power needed to propel the vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine.
HEV's also capture and store energy when the vehicle is experiencing powertrain braking. The electric motor functions as a generator when powertrain braking is performed. When the electric motor functions as a generator, the generator absorbs energy generated as a result of the powertrain braking and converts the energy to electric energy which may be used to increase the state of charge of a battery. Regenerative powertrain braking may be desirable beyond recouping energy because the hybrid electric vehicle's energy may be dissipated with minimal or no negative torque supplied by the frictional wheel brakes—thus reducing wear on the frictional wheel brakes.
During vehicle operation, the HEV may require frictional braking when the driver's braking demand exceeds the powertrain braking capability or when the battery is already charged to capacity.